Terminal device, verification device, key distribution device, content playback method, key distribution method, and recording medium

Abstract

The terminal device 600 comprises: a read unit configured to read encrypted content and a content signature from a regular region of a recording medium device 700 , and to read a converted title key from an authorized region of the recording medium device 700 , the converted title key having been converted from a title key with use of a content signature generated by an authorized signature device 500 ; a title key reconstruction unit configured to generate a reconstructed title key by reversely converting the converted title key with use of the content signature read by the read unit; and a playback unit configured to decrypt the encrypted content with use of the reconstructed title key to obtain decrypted content, and to play back the decrypted content.

The invention claimed is: 
     
       1. A terminal device comprising:
 a read unit configured to read encrypted content and a content signature from a recording medium device, and to read a converted title key from a protected area of the recording medium device, the converted title key having been converted and calculated from a parameter of a title key with use of another parameter of the content signature generated by an authorized signature device; 
 a title key reconstruction unit configured to subsequently generate a reconstructed title key as an authorized title key by reversely converting the converted title key with use of the parameter of said content signature read by the read unit; and 
 a playback unit configured to decrypt the encrypted content with use of the reconstructed title key which is authorized to obtain decrypted content, and to play back the decrypted content. 
 
     
     
       2. The terminal device of  claim 1  wherein
 the converted title key has been generated from the content signature generated by the authorized signature device, a usage condition of the encrypted content, and the title key, 
 the read unit is further configured to read the usage condition from the recording medium device, and 
 the title key reconstruction unit generates the reconstructed title key by reversely converting the converted title key with use of the content signature read by the read unit and the usage condition. 
 
     
     
       3. The terminal device of  claim 2  wherein
 the converted title key has been generated as a result of a predetermined calculation performed on first concatenated data and the title key, the first concatenated data being obtained by concatenating the content signature generated by the authorized signature device and the usage condition, and 
 the title key reconstruction unit generates the reconstructed title key, by generating second concatenated data from the content signature read by the read unit and the usage condition, and performing an inverse calculation of the predetermined calculation on the second concatenated data and the converted title key. 
 
     
     
       4. The terminal device of  claim 3  further comprising
 a content signature verification unit configured to judge whether the content signature read by the read unit matches the content signature generated by the authorized signature device, and if judging negatively, to prevent processing by the playback unit. 
 
     
     
       5. The terminal device of  claim 3  further comprising
 a content verification unit configured to verify, with use of the content signature read by the read unit, whether the encrypted content is authorized, and if judging negatively, to prevent processing by the playback unit. 
 
     
     
       6. The terminal device of  claim 3  wherein
 the content signature read by the read unit includes an identification information piece for identifying a signature device that has generated the content signature, and 
 the terminal device further comprises: 
 a reception unit configured to receive a revocation list including one or more identification information pieces respectively identifying one or more devices that have been revoked; and 
 a revocation check unit configured to check whether the signature device has been revoked with use of the revocation list, and if judging affirmatively, to prevent processing by the playback unit. 
 
     
     
       7. The terminal device of  claim 6  wherein
 the content signature read by the read unit further includes a first date information piece indicating a date on which the content signature was generated by the signature device, 
 the revocation list further includes one or more second date information pieces associated one-to-one with the one or more identification information pieces respectively identifying the one or more revoked devices, each second date information piece indicating a date on which the corresponding device was revoked, and 
 the revocation check unit 
 judges that the signature device has been revoked if the identification information piece of the signature device is included in the revocation list and the date indicated by the first date information is the same as or more recent than the corresponding second identification piece, and 
 judges that the signature device has not been revoked if the identification information piece of the signature device is included in the revocation list and the date indicated by the first date information is older than the corresponding second identification piece. 
 
     
     
       8. A verification device comprising:
 a read unit configured to read encrypted content and a content signature generated by an authorized signature device from a recording medium device, and to read a converted title key from a protected area of the recording medium device, the converted title key having been converted and calculated from a parameter of a title key with use of another parameter of the content signature generated by an authorized signature device with use of a content signature key, wherein a reconstructed title key can be subsequently generated as an authorized title key by reversely converting the converted title key with use of the parameter of said content signature; and 
 a content signature verification unit configured to judge whether the content signature read by the read unit matches the content signature generated by the authorized signature device. 
 
     
     
       9. A key distribution device comprising:
 a content holding unit configured to hold therein encrypted content that is content encrypted with use of a title key; 
 a content signature holding unit configured to hold therein a content signature generated by an authorized signature device for verifying whether the encrypted content is authorized; 
 a title key holding unit configured to hold therein the title key; 
 a key generation unit configured to generate a converted title key by converting the title key with use of a parameter of the content signature, wherein a reconstructed title key can be subsequently generated as an authorized title key by reversely converting the converted title key with use of the parameter of said content signature; and 
 a recording unit configured to record the encrypted content, the content signature, and the converted title key into a recording medium device. 
 
     
     
       10. The key distribution device of  claim 9  further comprising
 a usage condition holding unit configured to hold therein a usage condition of the content, wherein 
 the key generation unit generates the converted title key from the content signature, the usage condition, and the title key. 
 
     
     
       11. The key distribution device of  claim 10  wherein
 the key generation unit generates the converted title key by performing a predetermined calculation on concatenated data and the title key, the concatenated data being obtained by concatenating the content signature and the usage condition. 
 
     
     
       12. A content playback method used in a terminal device, comprising the steps of:
 reading encrypted content and a content signature from a recording medium device, and reading a converted title key from a protected area of the recording medium device, the converted title key having been converted and calculated from a parameter of a title key with use of another parameter of the content signature generated by an authorized signature device; 
 generating subsequently a reconstructed title key as an authorized title key by reversely converting the converted title key with use of the parameter of said content signature read by the read unit; and 
 decrypting the encrypted content with use of the reconstructed title key which is authorized to obtain decrypted content, and playing back the decrypted content. 
 
     
     
       13. A non-transitory computer-readable recording medium storing thereon a computer program used in a terminal device, the computer program causing the terminal device to perform the steps of:
 reading encrypted content and a content signature from a recording medium device, and reading a converted title key from a protected area of the recording medium device, the converted title key having been converted and calculated from a parameter of a title key with use of another parameter of the content signature generated by an authorized signature device; 
 generating subsequently a reconstructed title key as an authorized title key by reversely converting the converted title key with use of the parameter of said content signature read by the read unit; and 
 decrypting the encrypted content with use of the reconstructed title key which is authorized to obtain decrypted content, and playing back the decrypted content. 
 
     
     
       14. A key distribution method used in a key distribution device, the key distribution device holding therein:
 encrypted content that is content encrypted with use of a title key; 
 a content signature generated by an authorized signature device for verifying whether the encrypted content is authorized; and 
 the title key that encrypts said content, and 
 the key distribution method comprising the steps of: 
 generating a converted title key by converting the title key with use of a parameter of the content signature, wherein a reconstructed title key can be subsequently generated as an authorized title key by reversely converting the converted title key with use of the parameter of said content signature; and 
 recording the encrypted content, the content signature, and the converted title key into a recording medium device. 
 
     
     
       15. A non-transitory computer-readable recording medium storing thereon a computer program used in a key distribution device,
 the key distribution device holding therein: 
 encrypted content that is content encrypted with use of a title key; 
 a content signature generated by an authorized signature device for verifying whether the encrypted content is authorized; and 
 the title key that encrypts said content, and 
 the computer program causing the key distribution device to perform the steps of: 
 generating a converted title key by converting the title key with use of a parameter of the content signature, wherein a reconstructed title key can be subsequently generated as an authorized title key by reversely converting the converted title key with use of the parameter of said content signature; and 
 recording the encrypted content, the content signature, and the converted title key into a recording medium device.